These studies utilize chemical lesioning techniques and in situ hybridization to trace dopaminergic pathways and ascertain the functional neuroanatomical distribution of receptor subtypes. In recent experiments, it has been demonstrated that the A2 adenosine and dopamine D2 receptor are colocalized to the same neurons and that a small population of these cells can be found as part of the striatonigral pathway. Further studies demonstrated that D2 but not D1 receptor subtypes are expressed in striatopallidal neurons. These results may have important implications for understanding the diverse physiological and pharmacological effects of drugs selective for D1 and D2 receptors as well as improving our apprehension of pathological conditions which effect the basal ganglia.